The Great Intelligence was a mysterious entity. Few sources could agree exactly whAt The Intelligence was. The Second Doctor described it as “a sort of formless, shapeless thing floating about in space like a cloud of mist, only with a mind and will.” The Intelligence was capable of possessing deceased bodies, controlling mechanised Robot Yeti, forming fear-dependent snow creatures, and was often prone to choosing individuals to do its bidding from a distance.
Contents
Nature

The Great Intelligence was an aspect of Sunyata, (Night of the Intelligence) an “inter-dimensional being” that resided within the Void, existing in many realities, but always ending up as the “Great Intelligence” and sharing the same basic attributes, although the Intelligences originated in various ways, including an immortal soul, and various Great Old Ones. (Legacies) According to this account, one imprint of the Great Intelligence was the ascended immortal soul which began life as James Lethbridge-Stewart and ended millennia later as Mahasamatman. (The Forgotten Son) Another imprint was originally from the universe before the Doctor’s; the being known as Yog-Sothoth and his brethren survived the end of their universe by passing through a parallel universe that ended one second after theirs. Shifting again allowed them to enter the current universe shortly after it began expanding. (All-Consuming Fire, Millennial Rites) A third imprint was a Great Old One. (AUDIO: The Roof of the World) Like the other Great Old Ones, it may have lost its body when it ran from the Fendahl. (White Darkness)

Another account suggested that The Great Intelligence was created when Walter Simeon began speaking to a snowman created with a low-level telepathic field. The snowman reflected his most dark beliefs and thoughts, and it eventually formed its own consciousness, becoming the “dream [that] outlive[d] the dreamer”. (The Snowmen)

History
Early history

According to one source, Yog-Sothoth was a member, and the military strategist, of a race of beings called the Great Old Ones, who were the equivalent of the Time Lords in a previous universe to the one the Doctor resided. They shunted themselves into a parallel universe to pass into the next universe. Yog-Sothoth discovered it had gained god-like powers and decided to try the various gambits and games it had only played on computers. Over the billennia, it mounted millions of campaigns against inhabited planets. It used the Hisk version of koalas on Hiskith and domestic animals equivalent to dogs on Danos. (Millennial Rites)

According to another source, the Intelligence had been exiled from another dimension, and was forced to wander the universe to find a body to possess. (Doctor Who and the Web of Fear)
The Snowmen

According to a third account, the Great Intelligence was formed when a young Walter Simeon discovered that a snowman he had built began to talk back to him. The snow itself featured a low-level telepathic field, causing it to respond to the thoughts and emotions of those around them. Because of this, the Intelligence was nothing more than Simeon’s own dark thoughts reflected back at him. As an adult, Simeon established the Great Intelligence Institute as the snow slowly swarmed to Earth.

In 1892, its presence was sufficient enough to consume mankind. After Simeon’s mind and memories were erased from after meeting the Intelligence, the Eleventh Doctor was surprised to see the Intelligence survived. The Doctor discovered that it had learned to survive beyond physical form. Controlling the now-mindless Simeon, it attacked the Doctor, but was stopped in the last minutes of Christmas Eve when the snow changed to “rain”, mimicking the form of the tears of Captain Latimer’s family after Clara Oswin Oswald’s death.

The Intelligence did not totally perish, and in fact learned to survive beyond physical form. It continued with the belief that The London Underground was a key strategic weakness in the London metropolitan area. (The Snowmen)

The Yetis

At some point in the 18th century, it possessed the Tibetan lama Padmasambhava while it was travelling the astral plane and forced him to build its Robot Yeti over the next two centuries. (The Abominable Snowmen) According to one account, it was Padmasambhava that called it a “great intelligence”, a name it used from that point on. (The Forgotten Son)

At the Det-Sen Monastery in 1935, the Second Doctor and Edward Travers, a westerner determined to find the Yeti, intervened with the Intelligence’s plans to take the mountain the Monastery stood on, the Lama realising that it would want to cover the world as slime. The Intelligence knew of the Doctor, and described him as “a man of great knowledge and intelligence.” They destroyed the pyramids that controlled its Yetis and Padmasambhava’s physical body died as the Intelligence melted away. (The Abominable Snowmen)

The London Event

Now under pressure from the other Old Ones, who themselves embarked on similar campaigns and conquered other planets, the Intelligence was forced to use the Yeti in London, an environment to which they were not well suited. (Millennial Rites)

Approximately 40 years after the events At The Det-Sen Monastery the Yeti re-activated and the Intelligence manifested as webbing. It ensnared the Doctor’s TARDIS in space and forced it to land in the London Underground. Reunited with Travers, the Doctor assisted British military in their battles with the Yeti. The Intelligence re-animated and possessed the corpse of Staff Sergeant Arnold, using him to track the Doctor’s actions. The Intelligence captured the Doctor and tried to use a conversion headset to take over the Doctor’s body. The Doctor attempted to reverse the process, allowing him to absorb the Intelligence and destroy it. When the control spheres that formed the focus of the Intelligence were smashed by Jamie McCrimmon, the Intelligence vanished, (The Web of Fear) powerless but still alive, though blinded in unending darkness. (HOMEVID: Downtime)

In October of that same year, the Intelligence followed Lethbridge-Stewart, Sally Wright, and Owain Vine to New York City, where it attempted to use Yeti and infected Rats to turn the population of the city into a vast computer. Aided by Edward Travers and Adrienne Kramer, the three successfully repelled the invasion. (Times Squared)

Further exploits

In the 1980s, the Intelligence again used the Yeti in Tibet to attempt world conquest, only to be defeated by the Lama Gampo upon calling the real Tibetan Yeti to fight the machines. (Yonder… The Yeti)

The Intelligence later contacted the Doctor’s former companion Victoria Waterfield and manipulated her into using computers to return to physical existence in 1995, attempting to cover the Earth in web. When the generators to the New World University were destroyed, the main bulk of the Intelligence faded away. (HOMEVID: Downtime)

The Seventh Doctor, Lysandra Aristedes and Sally Morgan encountered the Great Intelligence during their travels in the black TARDIS. (AUDIO: Black and White)

The new millennium

In 1999, Anne Travers, who had been left traumatised by the Intelligence’s first attempts to enter the universe, believed millionaire Ashley Chapel would try to use a special program, the Millennium Codex, to summon the Intelligence to Earth. She prepared a counterspell to force it back into its own reality. However, this had destructive effects, including dragging the Intelligence back to Earth and merging it with the benevolent god Saraquazel into a single malevolent being.

It altered reality around London to form the Great Kingdom, a realm partly obeying the laws of the Intelligence’s universe, N-Space, and Saraquazel’s universe; here, the Doctor’s TARDIS was worshipped as “the Lady TARDIS” and the Intelligence was worshipped as “the key and the guardian of the gate,” forming the triad of gods with Saraquazel. Anne sacrificed herself to fix reality, but instead of destroying the Intelligence, she banished it. The Intelligence became stranded on the edge of the universe, riding the blue shift outwards into infinity. (Millennial Rites)

Losing its memory

According to one account, the Intelligence soon found itself on Earth in the year 1842. There it met a boy, who he connected to as the boy became a man. The Intelligence grew stronger, but still felt gaps in its memory covering centuries. It chose the name Great Intelligence, as given to him in Tibet. He continued to use the man’s form after his death, lingering for a hundred years in search of minds to use for its own. (The Forgotten Son)
Wi-Fi

At some point in the late 20th century, while Miss Kizlet was still a young girl, the Great Intelligence began to “whisper in her ear,” leading her to eventually found her company. By 2013, the Great Intelligence had influenced Miss Kizlet to establish an organisation based on the 65th floor of the Shard, which used the Wi-Fi and servants nicknamed Spoonheads to capture human minds. The Great Intelligence used the internet as its “web.”

Its operation fell apart when the Eleventh Doctor used a captured Spoonhead to trick Miss Kizlet into being trapped in the Wi-Fi after she refused to release the uploaded Clara Oswald, who was under the Doctor’s protection. The workers downloaded Clara, Kizlet and others captured in the server, returning them to their bodies.

When UNIT arrived At The Shard, the Great Intelligence ordered Kizlet to restore their employees to their “factory settings”, effectively erasing everyone’s memories to avoid detection. (The Bells of Saint John)

Entering the Doctor’s timestream

After the Siege of Trenzalore, (The Time Of The Doctor) the Intelligence used the Whisper Men to kidnap Madame Vastra, Strax and Jenny Flint from 1893 to Trenzalore. It spoke through their “conference call” link with Clara and River Song to take the Doctor from 2013 London to Trenzalore.

The Whisper Men brought the Paternoster Gang to the Doctor’s tomb on Trenzalore so the Intelligence could have its ultimate revenge by turning all of the Doctor’s victories into defeats. It did so by directly entering the Doctor’s time stream, which appeared as an open wound in reality inside the tomb. However, this plan was foiled by Clara, who followed the Intelligence through the wound. Just as he was, she was ripped into countless versions of herself throughout history, and saved the Doctor countless times, undoing the damage the Intelligence had done to his timeline. (The Name of the Doctor)

Targeting the Brigadier

After interfering with countless points in the Doctor’s timeline, the Intelligence returned to the London Event where another encounter with Clara, and the Second Doctor’s attempt to destroy the younger Intelligence, severely damaged it, forcing the Intelligence to leave the Doctor’s timestream. Realising that Clara would be waiting at every turn to defeat it, the weakened Intelligence travelled down the timeline of Alistair Lethbridge-Stewart instead, intending to kill the Doctor’s greatest ally when he was a boy.

The Intelligence arrived in 1937 and found itself drawn instead to Alistair’s brother, James, who carried the original soul which would, centuries later, be reincarnated as the being who would become the Great Intelligence. Its curiosity drew itself to join with James, but it then found that it could not remove itself from its own soul. After manipulating James into killing himself, the Intelligence became trapped inside Remington Manor in Cornwall.

Weeks after the original London Event, in 1969, the Intelligence used Owain Vine, another reincarnation of the Intelligence’s immortal soul. A final showdown in Remington Manor between the Intelligence and Lethbridge-Stewart saw the Intelligence finally defeated. The last trace of it permeated the ruins of Remington Manor. (The Forgotten Son)

Alternate timelines

In an alternative timeline the London Event succeeded, and both Colonel Lethbridge-Stewart and the Doctor were killed in the new timeline. Without them to stand against it, the Intelligence spread its influence throughout Southern England, bringing thousands of minds into its own. The TARDIS, taking on human form, revealed the truth of the Intelligence’s being, that it was the imprint of a multi-dimensional being, and a stalemate was called. The Intelligence could never win as long as the TARDIS existed, and the TARDIS could never restore time unless the Intelligence was cast out. Not wishing to continue the stalemate, the Intelligence allowed the TARDIS to cast it out into the void, and time was restored. (Legacies)

Parallel universes

On a parallel Earth, the Great Intelligence launched an attack on the Republic of Great Britain. Koschei, here having not become the Master, saved the world from the Intelligence but was then captured, tortured, and pumped for information for years by the Republican Security Forces. (The Face of the Enemy)

Personality

The Great Intelligence was arrogant and thought very highly of itself, informing the Doctor that his brain was too small to grasp its purpose. (The Abominable Snowmen) It sometimes spoke in the plural, i.e. referring to itself as “we”, although this habit had worn off by the time of the London Event. (The Abominable Snowmen, The Web of Fear, The Snowmen) Without a body, it became obsessed with having physical form, craving symmetry of light, colour, and shape. (The Snowmen) Eventually, it grew tired of living as “a mind without a body” and went into the Doctor’s timestream in order to die. (The Name of the Doctor)

Despite once describing revenge as a petty emotion of which it had no need for, (The Web of Fear) the Intelligence took its defeats by the Doctor as unforgivable wounds to its pride and after several encounters with him, it had developed such a burning hatred that it went into his timestream in order to take revenge on him in the most painful way imaginable. (The Name of the Doctor) It was also a malevolent and sadistic being, (The Abominable Snowmen, The Snowmen) and was also extremely callous and self-absorbed, seeing as how it was unfazed by the fact that undoing all of the Doctor’s victories would in turn cause the destruction of the universe. (The Name of the Doctor)

It was the unseen but generous Chancellor of New World University, with Victoria as Vice-Chancellor. It designed invasions based on a “Great Plan” it had, having been the military strategist of the Old Ones, and when it thought resistance was useless for humanity, it asked Brigadier Lethbridge-Stewart which part of its brilliant plan he found most effective. (HOMEVID: Downtime) When horribly bound to Saraquazel, it did not care about him or crave his assistance, thinking it possessed enough power and intellect to find its own escape. The Intelligence, however, did warn Saraquazel of the duplicity of human beings. (Millennial Rites)

Miss Kizlet believed that The Intelligence loved humanity in the same vein that Burger King loved cattle. The Intelligence was unconcerned about feasting on people’s minds to grow stronger, and showed little concern for its servants, reverting them back to their original state of mind, although it did take the time to say goodbye to Kizlet before doing so. (The Bells of Saint John)

Powers

The Great Intelligence had no physical existence and thus relied on possession of living creatures, (The Abominable Snowmen, The Web of Fear) taking on other shapes, (The Name of the Doctor) or manipulating allies in order to manipulate its environment. (The Snowmen, The Bells of Saint John)

Existing on the astral plane, the Intelligence could enter the people it encountered, filling itself in every cell and having control of their every movement. It also increased Padmasambhava’s lifespan, and reanimated the dead body of Staff Sergeant Arnold. (The Abominable Snowmen, The Web of Fear) At full strength it could even save a human from fatal illness, or death by extreme age, if only to possess their bodies. (Downtime)

In addition, it could possess animals, like rats and ants, by extending its will, as well as inhabiting machinery like computer terminals. It resided in the New World University network while reaching out into the Internet. However, the majority of its consciousness was still trapped in the campus mainframe. (HOMEVID: Downtime) It also inhabited a computer system with a video monitor linked to the firm run by Miss Kizlet in the Shard. The peripheral units linked to this system could edit or absorb the mental traits of users, enabling Kizlet to feed the Intelligence. (The Bells of Saint John)

It could also exert a mental control similar to hypnosis without completely possessing a human, as it had with the monks in Tibet, Victoria Waterfield, Walter Simeon and Miss Kizlet. (The Abominable Snowmen, The Snowmen, The Bells of Saint John) It also had considerable mental powers, namely telekinesis. (The Abominable Snowmen) It was able to make a Eurotrain rear up like a snake and launched missiles, playing with them and engineering near-misses before letting them fall. (Downtime) While merged with Saraquazel, it telekinetically constructed robot Yetis from its surroundings to defend itself. (Millennial Rites)

When the Paternoster Gang encountered the Intelligence on Trenzalore, it displayed considerable knowledge of the Doctor’s past and future, listing people he had killed, as well as names he’d have by the end of his life, including the Valeyard. When Madame Vastra asked how it had come by this information, it merely replied that it was information. (The Name of the Doctor) It also tracked the Second Doctor and followed him through time and space, having built a machine that could drain his mind of knowledge and experience for the Intelligence. (The Web of Fear)

Biology and appearance

“Was it huge with massive claws to crush and maim? A bloated spider-mind filling every cavernous gap with billowing web? Was it a mountain? A bank of mountains looming and rumbling like clouds in another sky or on another continuum?”
The Intelligence ponders its original form [src]
The Second Doctor thought the best way to describe the Great Intelligence was as a “formless, shapeless thing, floating out in space like a cloud of mist, only with a mind and will.” (The Web of Fear) Both Alistair Gordon Lethbridge-Stewart and the Eleventh Doctor identified it as a mind parasite, whereas the Intelligence considered itself a mass of thoughts with a single thought. (HOMEVID: Downtime; The Snowmen) It once reflected on whether or not it remembered what its original body was. (Downtime)
In all realities, the Intelligence constantly sought physical existence to replace being a shapeless, formless cloud hanging in space, (Legacies) eventually adopting Walter Simeon as a recurring avatar, speaking in that guise through a large wall-mounted video screen, (The Bells of Saint John) modelling the Whisper Men on Simeon’s appearance, (The Name of the Doctor) and appearing as Simeon to James Lethbridge-Stewart and Colonel Lethbridge-Stewart. (The Forgotten Son)

When not using a living being, the Intelligence maintained a basic manifestation as a three-sided pyramid composed of control spheres (The Abominable Snowman) or ivory. (HOMEVID: Downtime) When forcibly summoned to Earth by Anne Travers, and being combined with three sets of physical laws in the Great Kingdom, the Intelligence was an emerald tetrahedron and, because of Travers’ meddling, was merged with the god Saraquazel. (Millennial Rites)

The Eleventh Doctor commented that The Intelligence was a hive mind, (The Bells of Saint John) which accounted for the “mass of thoughts” theory, (HOMEVID: Downtime) while the Intelligence claimed itself as an embodiment of information, and Jenny Flint described it as “a mind without a body”. (The Name of the Doctor)

The Great Intelligence possessed some amount of artron energy, (Millennial Rites) and could consume the mental energy of humans to grow stronger. (The Bells of Saint John)

In its many attempts to achieve form, the Intelligence tried to manifest as ice people based on the human form, (The Snowmen) and a slime that glowed brightly with a piercing light. (The Abominable Snowmen) It also manifested as a dense fog that consumed anything entering it, and a poisonous fungus which spread through the London Underground. (The Web of Fear) A fourth invasion of Earth had it trying to perpetuate itself in every machine and being, with the whole planet cocooned in web. (Downtime)

At the end of its life, the Intelligence had never found substance, but rather used the Whisper Men, who appeared as faceless humanoids dressed just as Walter Simeon was when he died, to manifest in empty bodies. (The Name of the Doctor) The Whisper Men told Clarence DeMarco that They were a manifestation of the Intelligence when they revealed the location of the Doctor’s tomb to him. (Clarence and the Whispermen)

Behind the scenes

Yog-Sothoth is the name of a fictional deity created by H. P. Lovecraft, first appearing in The Case of Charles Dexter Ward. It was implied to be one of the most powerful beings in the universe (second to only Azathoth), the key and the gate through which the Old Ones entered the world. Its appearance is often a mass of spheres and it is described as being imprisoned outside of the space-time continuum in a place where it exists at every point in time and space.
Whether the Great Intelligence should be referred to as “it” or “he” is perhaps best left to personal preference. As the Intelligence has no physical body, it is doubtful as to whether it can be said to have a gender. Although a male face resembling that of Walter Simeon appeared to Miss Kizlet on a large wall-mounted video screen, and it spoke with a male voice in Victorian England and on Trenzalore, this could have been a favoured avatar rather than a gender choice.
Writer Neil Gaiman disclosed in Doctor Who Magazine [which?] that earlier drafts of his script for The Doctor’s Wife implied that House, the villain of that story, was actually the Great Intelligence. These hints did not make it into the episode as aired. The idea of the Great Intelligence as a villain for the revived series Doctor Who would later lead to The Snowmen.
With a gap of 44 years, the Great Intelligence holds the record for longest period of time between televised Doctor Who appearances.
Unusually, unlike other monsters that have appeared in both the classic and revived series, the original creators of the Great Intelligence (Mervyn Haisman and Henry Lincoln) have never been credited At The end of an episode of the reboot.